The present invention relates, in general, to electronics, and more particularly, to semiconductor devices.
With the increasing growth of wireless communications, radio frequency (RF) amplifiers are finding use in many new and varied applications. In the past, radio frequency (RF) amplifiers typically included a high frequency power transistor which was operated in a non-saturated mode to drive a load such as an antenna. In the ideal situation, the impedance of the load was perfectly matched to the output impedance of the amplifier. However, in almost all cases the load was not perfectly matched. Various other events could also cause a load mismatch. Some examples of such events include a service technician decoupling the load to service equipment or cables being severed by construction work and other events. These mismatches caused the output voltage on the amplifier output to increase. Typically, such increases could result in the output voltage increasing past the breakdown voltage of the high frequency power transistor thereby damaging the transistor. In order to minimize the damage, passive elements, such as capacitors and inductors, were externally connected to the output in a effort to reduce the voltage coupled back to the amplifier. These passive elements typically had detrimental effects on the operating parameters of the amplifier. Such passive elements dissipated power and resulted in higher power dissipation by the amplifier. Additionally, the passive elements could reduce the power gain, efficiency, and linearity of power the amplifier. Further, the passive elements increased the cost of the system containing the amplifier.
In other cases, diodes were externally connected to the output in parallel with the load in an effort to minimize damage to the amplifier. The diodes had to have a power dissipation that was sufficiently large enough to dissipate all the power that was presented to the amplifier output. Consequently, the diodes were large and had parasitic elements that also detrimentally effected the amplifier""s performance. The diodes also increased the amplifier""s costs.
Accordingly it is desirable to have a method of protecting an amplifier from excessive voltages on the output without degrading the performance of the amplifier.